Study of the physical origin of a resonant feedback in random lasers

By expanding the spatial distribution of a dielectric constant in random media into Fourier series, we analyze the assumption that the resonant feedback is provided by the presence of components with spatial periods satisfying the condition of the Bragg diffraction. Numerical simulations of random media allow us to predict specific features of eigenfrequencies of lasing modes, which are similar to those obtained in the literature from the numerical solution of the Maxwell equations. Thus, a plausible explanation of the physical nature of frequency selection in random lasers is revealed. Furthermore, using a simplified description of the Bragg feedback, we show why the threshold gain decreases with the increase in the sizes of the active region.